A. Field of the Invention
This invention relates to an improved system and process for alkylating aromatics using a fluidized bed reactor. The improved results of this invention are realized by injecting at least a portion of the alkylating reagent downstream from the location where the aromatic reactant is introduced. This can be accomplished, for example, by directly introducing the alkylating reagent into the fluidized bed, preferably at plural stages along the flow axis of the reactor. Alternatively, the alkylating reagent can be injected into a region between two separate and discrete fluidized beds. Preferably, a portion of the alkylating reagent also is introduced along with the aromatic reactant, or at least near the location where the aromatic reactant is introduced.
The processes and systems according to the invention can be used for any suitable aromatic alkylation reaction. This invention is particularly well suited for use in a process for producing xylene (preferably para-xylene) from toluene and methanol. When the invention is used in this procedure, a significant improvement in toluene conversion, methanol selectivity, and selectivity to para-xylene can be realized. The invention also can be used, for example, in the production of other alkylaromatics including, for example, ethylbenzene, cumene, diethylbenzene, diisopropylbenzene, para-ethyltoluene, para-cymene, and pseudocumene, and for the reduction of benzene in motor fuels.
B. Description of the Prior Art
Aromatic alkylation is an important procedure for producing many useful chemical products. For example, para-xylene, which can be produced by alkylating toluene with methanol, constitutes an important starting material for manufacturing terephthalic acid, which is an important intermediate in production of synthetic polyester fibers, films, or resins. These polyester materials have many practical, well known uses, such as in fabrics, carpets, and apparel. Ethylbenzene, which can be produced by alkylating benzene with ethylene, is used mainly as a precursor for styrene production. Styrenes and polystyrenes are well known for many uses and products, including: packaging and disposable serviceware associated with the food industry; audio and visual cassettes; medical and dental molding products; and synthetic plastics, rubbers, and foams.
Because of the importance of alkylated aromatic products as starting materials and intermediates for producing many common consumer and industrial products, efficient production and use of alkylated aromatics is of great importance. Additionally, most aromatic starting materials, such as toluene and benzene, are obtained during oil and gas production. Therefore, efficient alkylation of these aromatic materials is vital to eliminate wastes and conserve precious natural resources.
A conventional approach for toluene alkylation includes mixing toluene and methanol upstream of a reactor and then feeding the mixture together into the base of the reactor. The reactor includes an alkylation catalyst in one or more fixed beds, and this catalyst promotes the alkylation reaction between the toluene and methanol to produce xylene. While this approach has been used successfully, its yield and reactant utilization characteristics leave room for improvement.
In an effort to improve yields in various reaction procedures, stagewise injection of reagents has been used in various fixed bed processes. For example, U.S. Pat. Nos. 4,377,718 and 4,761,513 describe toluene alkylation processes wherein the alkylating reagent is fed at different stages between fixed beds. Likewise, U.S. Pat. No. 3,751,504 discloses a similar procedure, using multiple injection ports, for preparing ethylbenzene using a fixed bed catalyst reactor. U.S. Pat. No. 5,120,890 discloses multiple reactant injection locations into separate fixed beds in a process for reducing benzene and toluene content in light gasoline streams. U.S. Pat. Nos. 3,751,504; 4,377,718; 4,761,513; and 5,120,890 are each entirely incorporated herein by reference.
In these fixed bed processes, one can easily separate the catalyst load into several different and discrete zones. During use, product from one zone is mixed with additional alkylating reagent, and this mixture is fed to the subsequent zone. One way of providing these separate and discrete zones includes placing each zone in a separate reactor vessel, wherein additional reagent(s) is (are) injected between adjacent zones. This procedure suffers from the drawback that considerable expense is involved in providing separate reactor vessels and.the associated hardware for running this type of system.
Additionally, fixed bed reactors are disadvantageous for exothermic reactions because of the potential negative impact of exotherms on product selectivity. Reactor stability concerns with fixed beds also require that the temperature rise per catalyst bed be limited. This could necessitate a large number of beds to accommodate the heat of reaction. Similarly, endothermic reactions would result in reduced reaction rates and excessive catalyst requirements.
It is an object of this invention to provide processes and systems for alkylating aromatic reactants with high conversion and selectivity, e.g., for producing para-xylene from an alkylating reaction between toluene and methanol: In general, the processes and systems according to this invention use stagewise injection of the alkylating reagent (e.g., methanol) at a location downstream from the location where the aromatic reactant is initially introduced into the fluidized bed.
A first aspect of this invention relates to a process for alkylating an aromatic reactant (e.g., methylating toluene to xylene). The process includes introducing the aromatic reactant (e.g., toluene) into a fluidized bed reaction zone, wherein the fluidized bed reaction zone includes a top portion, a bottom portion, and an intermediate portion extending between the top portion and the bottom portion. The alkylating reagent (e.g., methanol) is introduced directly into the intermediate portion of the fluidized bed reaction zone, where it reacts with the aromatic reactant (e.g., toluene) to produce the alkylated aromatic product (e.g., xylene). This product is recovered from the fluidized bed reaction zone.
In addition to introducing alkylating reagent directly into the intermediate portion of the fluidized bed reaction zone, alkylating reagent also may be introduced into the bottom portion of the fluidized bed reaction zone. This additional alkylating reagent can be introduced in a common feed stream with the aromatic reactant, or it can be separately fed into the fluidized bed reaction zone.
One preferred embodiment of the process according to the invention includes. introducing alkylating reagent directly into the intermediate portion of the fluidized bed reaction zone at a plurality of locations. These plural locations preferably are provided at a plurality of different axial positions in the intermediate portion of the fluidized bed reaction zone. Also, the alkylating reagent can be introduced at a plurality of different locations in the plane perpendicular or substantially perpendicular to the axial direction of the reactor vessel (i.e., at plural locations at each stage of its introduction).
Another embodiment of the process according to the invention includes the use of two or more fluidized bed reaction zones arranged in series. Preferably, the aromatic reactant is introduced into a first fluidized bed reaction zone of the series, i.e., the zone located furthest upstream. The alkylating reagent may be introduced into a region of the reactor system between the first fluidized bed reaction zone and a second fluidized bed reaction zone. During operation, the aromatic and alkylating reagents react to produce the alkylated aromatic product, which then is recovered from the reactor system.
In this embodiment of the process according to the invention, the first and second fluidized bed reaction zones (or more) can be contained either in a single reactor vessel or in a plurality of reactor vessels arranged in series with respect to the aromatic reactant flow. Preferably, in this embodiment, the alkylating reagent is introduced into the reactor system at a bottom of the second fluidized bed reaction zone (e.g., at the bottom of the second reactor), and, if more than two fluidized bed reaction zones are provided, preferably the alkylating reagent is introduced between each adjacent pair of the reaction zones. As with the first embodiment, if desired, additional alkylating reagent can be introduced into the bottom portion of the first fluidized bed reaction zone, optionally in a common feed stream with the aromatic reactant.
The invention also relates to systems for alkylating an aromatic reactant to an alkylated aromatic product. In this aspect of the invention, a reactor system provides at least a first reactor vessel that contains at least a first fluidized bed reaction zone. An appropriate means is provided for introducing the aromatic reactant into the reactor system at a first location in the first reactor vessel, preferably at the bottom of the most upstream fluidized bed reaction zone A means also is provided for introducing the alkylating reagent into the reactor system at a location downstream from the first location where the aromatic reactant is introduced. At least a portion of the alkylating reagent is introduced within the first fluidized bed reaction zone or at a location between the first fluidized bed reaction zone and a second fluidized bed reaction zone. The system also provides appropriate means for recovering the alkylated aromatic product.
In one preferred embodiment of the system according to the invention, the means for introducing alkylating reagent introduces this reagent at a plurality of locations spaced apart along an axial direction of the first reactor vessel. This reactor vessel may include one or more individual fluidized bed reaction zones. When plural fluidized bed reaction zones are provided, it is preferred that a means for introducing alkylating reagent provide the reagent between each adjacent pair of fluidized bed reaction zones in the series.
Accordingly, this invention provides processes and systems for producing alkylated aromatics (e.g., xylene) from an aromatic reactant (e.g., toluene) and an alkylating reagent (e.g., methanol), wherein the processes and systems provide high conversion, reactant utilization, and product selectivity, particularly when producing para-xylene from toluene and methanol.